The peroxisome is a ubiquitous organelle of eukaryotic cells Peroxisomes participate in a wide variety of metabolic processes and defects in peroxisome function re the cause of numerous human diseases. There are two general classes of peroxisomal disorders. The peroxisome biogenesis disorders include Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease and rhizomelic chondrodysplasia punctata and are characterized by defects in peroxisomal enzyme import and loss of virtually all peroxisomal functions. The single enzyme peroxisomal diseases are caused by defects in particular metabolic enzymes and affect only a single peroxisomal function. In this grant we will resolve outstanding questions regarding the peroxisome biogenesis disorders and thereby identify novel PEX genes. In aim 2 we will search for novel human PEX and PBD genes and solve the genetic basis of disease in three groups of PBD patients. In addition, we will continue our search for novel complementation groups of PBD patients. In aim 3, we will address the molecular mechanisms of disease in CG1 and CG4 of the PBDs, the two largest groups of Zellweger spectrum patients. These studies will improve our understanding of eukaryotic cell biology, metabolism, and the molecular basis of human disease.